Our lab studies mechanisms of neuronal development, focusing in particular on the visual and olfactory systems during embryonic and metamorphic development of the frog Xenopus laevis. In the past year we have made several important advances. We have shown that grol, a monoclonal antibodies generated in our lab, stains an antigen distributed in a pattern that suggests a role in optic nerve development. In the frog, the optic nerve projects to two main targets in the brain, but grol is only associated with one of the targets, suggesting that its differential distribution in the nerve could regulate axon routing. In addition if eyeless animals are made by deletion of optic placodes at embryonic stages, gro1 staining persists in the regions of the optic pathway normally occupied by nerve fibers. Thus grol immunoreactive cells associated with the brain appear to establish a "prepattern" which is followed by embryonic optic axons. We are in the process of identifying the grol antigen in the olfactory system, gro1 stains heavily in olfactory nerve and bulb, but staining stops abruptly at the caudal border of the olfactory bulb. At the point where grol staining ceases, staining of ipsl, a different antigen, is quite strong. Thus the grol/ipsl boundary defines the border of the olfactory terminal zone. We are analyzing the possibility that the early establishment of the olfactory terminal zone in telencephalon results from a repulsion of olfactory axons by the ipsl antigen. We are also performing placode grafts and deletions to determine how grol and ipsl expressing cells interact during the earliest stages of olfactory nerve development.